PLAB 1 (GMC, international medical graduates) · Statistics and evidence-based practice

Risk, study design and interpretation

8 practice questions, each with the answer and an explanation.

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  1. In a trial, 12 of 100 patients on the new drug and 20 of 100 on placebo had a stroke. What is the number needed to treat to prevent one stroke?
  2. In a trial, 12 of 100 patients on a new drug and 20 of 100 on placebo had a stroke. What is the relative risk of stroke on the drug compared with placebo?
  3. A case-control study of 100 cases and 100 controls finds that 40 cases and 20 controls were exposed. What is the odds ratio?
  4. A hospital measures the systolic blood pressure of 100 patients. The mean is 120 mmHg and the standard deviation is 15 mmHg. What is the approximate 95 percent confidence interval for the mean?
  5. A trial reports a relative risk of 0.8 with a 95 percent confidence interval of 0.6 to 1.1. How should this result be interpreted?
  6. What does a p-value of 0.03 mean in a randomised trial?
  7. In a study the researchers fail to detect a real difference between two treatments because the sample was too small. Which error has occurred?
  8. Which study design is most appropriate to investigate the causes of a rare disease?

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